1. Field of the Invention
The present invention generally relates to a fan filter unit, and in particular to a fan filter unit having sound-absorbing wedges for reducing noise caused by the operation of the fan.
2. Description of the Prior Art
Fan filter units (FFUs) are commonly incorporated in an air clean system for providing cleaned air flows to meet the severe requirement of low contamination for clean room environments. Besides operation efficiency and capability of removing contamination, an important concern of a fan filter unit the level of noise caused during the operation of the fan filter unit.
FIGS. 1 and 2 respectively show a top view and a cross-sectional view of a conventional fan filter unit. The fan filter unit comprises a housing 1 defining a bell mouth 3, serving as air inlet 11 of air flow A, and an opposite opening to which a filtering net 6 is mounted for serving as an air outlet 12 of the air flow A. A centrifugal fan assembly 2 is mounted in the housing 1 and coupled to the bell month 3. A guide plate 4 is positioned proximate the fan assembly 2 for guiding the air flow A. A baffle plate 5 is arranged between the guide plate 4 and the filtering net 6. The baffle plate 5 extends from an inside surface 13 of the housing 1 and defines an opening through which air flow passes toward the filtering net 6.
By means of the guide plate 4 and the baffle plate 5, a serpentine path of the air flow is formed inside the housing 1 which forces the air flow A to repeatedly contact the inside surface 13 of the housing 1 and the baffle 5. A sound-absorbing material is formed on the inside surface 13 of the housing 1 and the baffle plate 5 whereby the noise caused by the operation of the fan assembly 2 can be reduced due to the contact of the air flow A with the sound-absorbing material installed on the inside surface 13 of the housing 1 and the baffle plate 5.
Test and measurement show that the prominent noise caused by the fan assembly 2 occurs around the blade passage frequency of the fan assembly 2. Thus, low frequency noise around the frequency of 250-1000 Hz has a generally higher level among all the possible frequencies. However, the sound-absorbing material that is commonly employed in a fan filter unit is subject to the limitation of the configuration of the housing 1 whereby it cannot effectively reduce the low frequency noise.
Thus, it is desired to provide a low noise fan filter unit on which the noise level is effectively reduced.
Accordingly, an object of the present invention is to provide a fan filter unit having a low noise level during operation.
Another object of the present invention is to provide a fan filter unit incorporating wedge-shaped sound-absorbing elements for effectively reducing the level of noise caused by the operation of the fan.
A further object of the present invention is to provide a wedge-shaped sound-absorbing structure for a fan filter unit which effectively reduces low frequency noise level while keeping fluid flow resistance in a comparable range to the conventional configuration of the fan filter units.
To achieve the above objects, in accordance with the present invention, a fan filter unit comprises a housing having a side wall and a top wall with a bell mouth serving as an air inlet and an open bottom to which a filtering net is attached to serve as an air outlet. A centrifugal fan for driving the air from the inlet is arranged in the housing. A guide plate is arranged in the housing for guiding the air flow along the path from the fan assembly to the air outlet. A sound-absorbing structure including a plurality of wedge-shaped elements made of a sound-absorbing material is attached to the side wall. The wedge-shaped elements are arranged in a line with gaps formed between the wedge-shaped elements for effectively reducing noise level caused by the operation of the fan assembly. A second line of wedge-shaped elements is optionally attached to the side wall with the wedge-shaped elements of the two lines alternating each other.